Typical carburetor control valve assemblies have a throttle valve for control of engine speed and/or power, and a choke valve to facilitate cold engine starts. Often, the throttle valve is mechanically linked to the choke valve to further improve engine start reliability. Although less common, some control valve assemblies of a carburetor comprise only one valve, typically known as a throttling choke valve. For applications having both a choke valve and a throttle valve, the throttle valve typically has a metallic throttle lever for rotation of the throttle valve from slow idle to wide open positions. A stop is engaged by the throttle lever to restrict maximum opening of the throttle valve. Current choke valves have a metallic choke lever for manual or automatic rotation of the choke valve from full open for normal engine operation to substantially closed for cold engine starts. In some known applications, the metallic choke lever includes a cam surface that contacts a metallic cam member of the throttle valve typically engaged to an opposite end of the throttle shaft from the throttle lever.
The throttle lever, cam member and choke lever are attached to respective metallic throttle and choke shafts by either a threaded fastener or by swaging over an end portion of the shaft(s) extending through the levers and member. The throttle and choke valve heads are typically discs such as that of a butterfly valve located in the fuel-and-air mixing passage, and with the disks attached to the shafts by threaded fasteners. The machined or stamped components are relatively expensive to produce. Further, the use of threaded fasteners to connect the valve head and/or levers to the shaft, and/or the swaging of the shaft onto the levers greatly increases the cost, difficulty, time and labor required to assemble the control valve assembly in the carburetor body.
In some applications such as that taught in U.S. Pat. No. 6,708,959, issued Mar. 23, 2004, assigned to Walbro Engine Management, L.L.C., and incorporated herein by reference in its entirety, a plastic throttle lever connects to a plastic throttle shaft and the valve head or disc press fits into a longitudinal slot of the plastic shaft. As is commonly known in the art, a coiled torsion spring disposed axially on and engaged between the throttle lever and carburetor body rotationally biases the throttle valve toward an idle position. Although this design is known to reduce manufacturing costs by switching to snap-fitted plastic components, the natural tendency of the spring to expand axially as it resiliently coils when the throttle valve is rotated can loosen or dislodge the snap fitted throttle lever from the throttle shaft.